The invention relates generally to the field of imaging shallow formations and discrete seismic anomalies below the bottom of a body of water. More specifically, the invention relates to methods and apparatus for acquiring higher resolution images below the bottom of a body of water than is currently possible using devices known in the art.
Offshore geotechnical engineers preferably use reliable and representative borehole and/or cone penetrometer data for marine geotechnical (sub-bottom formation) assessment. The collection of these forms of samples, both physical or cone penetrometer-derived data is expensive and limited in its spatial representation of what is the true nature of the sub-bottom. In addition, the samples recovered may have been altered or biased during their collection. These possibilities introduce uncertainties, often putting into question the reliability of the physical boundaries displayed in cross-sections of the samples recovered or in the interpretation of the penetrometer profiles and values. Without additional verification and multiple or dense spot sampling, such geotechnical physical samples remain in effect a one-dimensional input to site investigations.
Both lateral and discrete discontinuities within the sub-bottom are important for offshore engineering and construction, risk mitigations. These discontinuities arising from complex geological sedimentary, tectonic, and glacial processes are also factors that influence the quality and dependability of physical core retrieval. These discontinuities in the sub-bottom can take the form of soft sedimentary lenses, boulder/cobble erratics, glacial tills, hard pans, fluidized discontinuous mud layers, gas hydrates, gas-charged sediments and periglacial frozen soil features. The resultant core samples may not be capable of capturing and retaining such materials or produce undisturbed samples thereof at the surface. Soft sediments become compressed during core extraction and depending on the extraction conditions may fluidize some of the materials in the core sample. This means an incorrect stratigraphic interpretation could occur but such would not be known or suspected when observing the core sample.
In addition it is well recognized in geophysical literature that imaging of lateral stratigraphy and showing only major sedimentary changes is not always indicative of the true stratigraphic organization of facies. Minor banding may be representative of true layering or such may be minor, highly localized internal sediment banding. In addition there may be disruptive blockages by large particulates or the inclusion of highly dense fragments. These blockages and or inclusions could be misinterpreted as belonging to a bedrock formation that does not exist at that particular depth. Without a multiplicity of physical cores taken in close proximity, the true lateral extent and nature of these boundaries may not be known.
To supplement physical sampling methods there is wide use of various geophysical acoustic and seismic based surveying techniques to map the dominant sub-bottom features' specular reflective properties. These techniques generally rely on having continuity in sub-bottom formation “horizons” or layers (i.e. acoustic/seismic boundaries). However, the sub-bottom is not composed only of laterally smoothly varying spatial features but also include widely distributed scattering and attenuating sedimentary and bedrock features or characteristics which translate into back-scattered, diffuse, non-specular acoustic reflections. Such diffuse reflections are generally interpreted as noise and subsequently filtered out of the data by conventional processing techniques. It is desirable that such sub-bottom texturally induced responses be ultimately captured and visualized in a coherent manner and interpreted in sub-bottom acoustic/seismic data.
Techniques known in the art cannot acoustically/seismically image the surrounding inhomogeneous sediment conditions such as required by offshore installation engineers and for pile or drill emplacements. Existing sonar techniques cross-reference poorly their data responses because such data are captured through continuously moving acoustic data acquisitions with sparse coverage with respect to the order of the wavelength of the intended features to be imaged.
Acoustic imaging methods known in the art, which are used extensively in connection with marine drilling, coring and in situ cone sensing remain isolated within their own prime physical interactions with the water sub-bottom. There has been a demand from the marine geotechnical community for better tools and more reliable correlations between the various data sets physically collected. Several developments have been noted in the domain of high resolution geophysics. See, for example, “Acoustic Sub Surface Interrogator (ASI), Guigné, 1990, U.S. Pat. No. 4,924,449 and, ‘Wide Area Seabed Analysis”, Guigné, 2010, U.S. Pat. No. 7,715,274. The foregoing patents describe three-dimensional mapping of geophysical parameters of the near sub-surface with greater accuracy and detail than has proven to be attainable using conventional seismic site surveying procedures
The ASI referenced above, includes various configurations of a positional transducer array on a stationary platform resting on the water bottom. The array coherently transmits signals with a signal having been specifically selected and programmed in terms of power, center frequency, beam-width, bandwidth, shape and incident angle. A positional receiver array on the platform captures the returned signals. Sub-surface acoustical properties, at the location of deployment, are identified through beamforming from various directional aspects within the platform footprint, then various geotechnical correlations are predicted from the processed returned signals. A calculation of the speed of sound in the sub-bottom, at the site of investigation, is introduced through the use of two extra orthogonal data line collections, which follow traditional seismic data acquisition routines involving time migration protocols. Subsequently, sub-bottom positions within a selected volume are then interrogated using well understood processing algorithms based on synthetic aperture sonar principles involving combined continuously gathered, successive transmissions coherently acquired along a precise data acquisition track in order to increase the azimuth (along-track) resolution. An interpretation is made of acoustical reflected and back-scattered properties between locations profiled, to develop a distribution model of the specular and diffused properties within a volume in the sub-bottom.
In addition there are numerous sub-bottom profilers on the market that utilize the well known and practiced art of synthetic aperture sonar, wherein independently of the acoustic signal generation and recording, there is an exacting and demanding continuous geodetic position requirement and orientation of the system for sub-bottom features to be coherently imaged on and be constructively produced. This continuous motion is typically measured using a high-grade inertial navigation system (“INS”) whose clock is synchronized with that of the acoustics sensors. The INS data are typically collected at the highest possible refresh rate, which typically ranges from 10 to 25 Hz. Without occasional input of absolute geodetic position information at selected time intervals, an INS system drifts during long term usage hence sonar profiling systems involving Synthetic Aperture Sonar architectures depend on and are fallible to their INS systems which periodically or continuously require corrections for drift.
There continues to be a need for high resolution sub-bottom imaging methods and apparatus.